The present invention relates to a a heat exchanger and, more particularly, to a plate heat exchanger, for exchanging heat between at least two fluids in an air separation unit.
An air separation unit comprises several types of heat exchanger.
A main heat exchanger is used to cool the feed air of the unit to the distillation temperature by exchanging heat with one or more fluids coming from the distillation unit. In certain cases, these are pressurized liquids from the unit which vaporize in exchange with the air to be distilled in the exchanger. These exchangers are normally made entirely of aluminium or copper or alloys of these metals (WO95/28610, Hausen, Linde xe2x80x9cTieftemperaturtechnikxe2x80x9d, pages 468-471, xe2x80x9cLarge Tonnage Oxygen Plantsxe2x80x94Brazed Aluminium Technology and Equipment for the 80sxe2x80x9d, Duncan et al., Cryogenic Processes and Equipment Conference, ASME, August 1980, xe2x80x9cImproved Plant Main Condenserxe2x80x9d, O""Neill et al., Cryogenic Processes and Equipment Conference, ASME, August 1980).
For safety reasons, these liquids are sometimes vaporized in a special exchanger in exchange with a single fluid such as air or nitrogen.
The unit also comprises at least one vaporizer-condenser, which is a heat exchanger placed inside or outside the column. These vaporizers are normally made entirely of copper, stainless steel, nickel or aluminium and consist of at least two circuits, including at least one which is connected to the rest of the plant by means of pipes welded to the equipment.
These exchangers usually comprise a plurality of aluminium plates separated from one another by between 5 and 7.6 mm (U.S. Pat. No. 4,715,433) with aluminium corrugations between the plates.
In EP-A-0 952 419, the distance between the aluminium plates is at most 5 mm.
At cryogenic temperatures, the thermal conductivity of copper is about three times higher than that of aluminium. The height of the corrugations (and therefore the distance between the plates) can hence be increased in order to improve the heat exchange and the number of plates will be reduced, as illustrated in FIG. 7. FIG. 7 shows the change in the effective area as a function of corrugation height for various materials and at different temperatures. The effective area corresponds to the primary area (separating sheets) plus the secondary area (corrugations) corrected by a fin coefficient.
According to one object of the invention, a plate heat exchanger is provided, comprising:
a plurality of metal plates made of copper, nickel, aluminium or an alloy comprising at least 80% copper, at least 80% nickel or at least 80% aluminium, which have a substantially similar contour, are parallel and are spaced apart from one another in order to form passages;
exchange corrugations comprising at least 80% copper, which are placed between at least two plates,
a closure means consisting of side bars connected to the edges of the plates in a leaktight fashion;
two outer sheets which are parallel to the plates and of a contour substantially similar to those of the plates;
optionally, semicylindrical heads connected to the passages between the plates,
and, optionally, a fluid inlet/outlet chamber connected at a leaktight joint to a fluid inlet or outlet face, at least some of the chamber consisting of at least one sphere or ellipsoid portion and of sections of cones tangential to this sphere or ellipsoid portion
characterized in that the distance between the adjacent edges of the plates, between which corrugations made of at least 80% copper are placed, is more than 6 mm, optionally 8 mm.
The distance between the adjacent edges of the plates is preferably more than 9 mm or 10 mm.
The thickness of the plates varies from 1 mm to 25 mm.
The thickness of the corrugations varies from 0.1 mm to 0.4 mm.
The frequency of the corrugations varies from 300 corrugations/meter to 1200 corrugations/meter.
All the corrugations of the exchanger are preferably made of copper.
The exchanger may fulfil one of the roles described above in an air separation unit.
For example, it may be the main exchanger which is used to cool the air to its distillation temperature or a subcooler.
If the unit comprises a first column which is fed with air and is thermally connected to a second column, a heat exchanger according to the invention may make it possible to heat the base of the second column with the head gas from the first column. Only two different flows circulate through the exchanger.
Alternatively, the heat exchanger according to the invention may be an intermediate exchanger of the second column or a head condenser of a single column.